Japanese, Farsi, Korean, Vietnamese, Thai, Turkish and other national languages don't have a strong position in the world, and even if the language of the poorer countries becomes more important as they develop, they'll can never compete with the real major languages (besides Japan and Korea are soon facing major democraphic bombs). So we have Arabic, Bahasa Indonesia, Russian and German left: if Russia manages to become a major power, than RUssian will grow, but I don't think they can do so before Africa and Asia rise and so they will at most be an important regional and a secondary world language. German is probably past its peak, although in a better position than Japanese and Russian because of the demographic issue, of being an European Latin alphabet based language. Arabic will probably grow a lot, but how much it can compete with the established major languages is still and issue and will depend on how much the regions where it is a native langauge can develop and solve their own internal issues. Bahasa Indonesia is in a great position to become a major regional language, but there is not much really going on for it to become an international language.
As with Chinese... it is hard to say, but I'd guess it will be more or less like Japanese in the eighties: a lot of people will say they want to learn it, some people will start studying it, but only very few people will acquire fluency and China will mostly conduct its business overseas in English, French and Spanish.